DOHA, April 4 (Reuters) - Qatar's top bank QNB is helping the country's authorities with data and expertise to probe if Qatar's neighbors manipulated its markets, including credit default swaps (CDS), during a political standoff last year, its CEO told Reuters.

Qatar's central bank said in December it had launched a legal investigation into attempts by countries opposed to it to harm the Qatari economy by manipulating the currency, securities and derivatives markets.

Qatar National Bank (QNB) Chief Executive Ali Ahmed al-Kuwari said in an interview there had been unusual moves in Qatari CDS late last year.

"The behavior was abnormal ... the way it was moving very fast, this is not normal behavior. The dust around the crisis settled after two months, but the CDS continued to go up for no reason," he said, estimating the Qatari CDS market at $21 billion.

Al-Kuwari said QNB was cooperating with the central bank in its investigation and had been "supplying them with any information or data they asked us to supply them with."

Saudi Arabia, the United Arab Emirates, Bahrain and Egypt imposed an economic boycott on Qatar last June, accusing it of backing terrorism, which it denies. The boycott led to the withdrawal of billions of dollars of deposits from Qatari banks.

Last month, Qatar's central bank asked U.S. regulators to investigate the U.S. unit of the largest bank in the United Arab Emirates, accusing it of "bogus" foreign exchange deals designed to harm its economy as part of a blockade by Gulf neighbors.

First Abu Dhabi Bank, parent of the U.S. subsidiary, NBAD Americas, denied it had tried to manipulate the Qatari riyal.

The request for a U.S. investigation deepened the diplomatic crisis between Qatar, a major gas exporter, and its Gulf neighbors.

Asked if there was concern about further attempts to manipulate Qatar's economy, al-Kuwari said: "This is a major offense. People will have to think about the consequences. And especially now with the legal case on the table, I assume people will think twice before going further into these actions."

Al-Kuwari said that despite a substantial withdrawal of deposits at the start of the Gulf crisis in the middle of last year, deposits at QNB grew 13 percent for the whole of 2017.

He said the trend will likely be sustained in the first quarter across the entire Qatari banking sector thanks to higher oil and gas prices and renewed confidence in the economy.

Al-Kuwari said QNB had repaid a cash injection it received from Qatar's central bank at the peak of the crisis last year.

He did not give the amount but said it represented a fair share among Qatari banks. He said the overall injection amounted to around $20 billion for the whole of the banking sector, half of which is controlled by QNB.

Qatar was able to beat the crisis partly thanks to a big cash buffer of $350 billion in central bank reserves and assets at the Qatar Investment Authority, which include stakes in major companies including Glencore and Volkswagen. (Editing by Adrian Croft)